Improve zsh completion

[kymnob]

• [ ] Perhaps a few more adjustments will be needed once void-linux/xmirror#16 and #590 are resolved.
• [ ] In the end, I'd like to exclude long-form options (--debug) if short one (-d) is already present on the command-line (and vice versa)
• [ ] and maybe lowercase all leading capital letters in descriptions.

Since we're parsing search output already.. Is there interest in a somewhat more verbose package completion? Like completing package names over the output of xbps-query -Rs ''. In other words, do you need some additional info during package name completion on a regular basis?